In general, ink jet printing machines or printers include at least one printhead that ejects drops or jets of liquid ink onto a recording or image forming media. A phase change ink jet printer employs phase change inks that are in the solid phase at ambient temperature, but transition to a liquid phase at an elevated temperature. The molten ink can then be ejected onto a printing media by a printhead directly onto an image receiving substrate, or indirectly onto an intermediate imaging member before the image is transferred to an image receiving substrate. Once the ejected ink is on the image receiving substrate, the ink droplets quickly solidify to form an image.
In both the direct and offset printing architecture, images may be formed on a continuous media web. In a web printer, a continuous supply of media, typically provided in a media roll, is conveyed by a plurality of rollers that are arranged to guide the media web through a print zone where a plurality of printheads are positioned to deposit ink onto the web to form images. Beyond the print zone, the media web is gripped and pulled by mechanical structures so a portion of the media web continuously moves through the print zone. Tension bars or rollers may be placed in the feed path of the moving web to remove slack from the web so it remains taut without breaking.
In continuous-web direct to paper printing, a fixing assembly is used after the ink is jetted onto the web to fix the ink to the web. The fixing assembly used depends on the type of ink. For example, when using melted phase change ink to form images, the fixing assembly may include a spreader configured to apply pressure to the ink and web to spread the ink on the web. The function of the spreader is to transform a pattern of ink droplets deposited onto a web and smear them out to make a more uniform and continuous layer. The spreader uses pressure and/or heat to reduce the height of the ink droplets and fill the spaces between adjacent drops. When UV curable inks are used, the fixing assembly may include one or more curing lamps to cure the UV ink onto the web.
Sometimes the ink deposited onto the web may bleed into the web before the ink is fixed to the web. For example, a liquid or molten uncured ink may bleed into the fibers of a paper substrate and become at least partially visible from the backside of the substrate. This problem is known in the art as showthrough or bleed-through, and is generally known to exist for any type of liquid ink deposited on a porous substrate. This issue is more pronounced in inks of low viscosity, such as ink jet inks, while higher viscosity inks are less susceptible to this problem. Specifically, showthrough is a measure of how colorized an ink makes the backside of the substrate.
In previously known systems, bleed-through detection on a temperature sensitive printing system (i.e. ink jet) was only able to be detected visually, after the image had been printed. In addition, the ability to correct or remediate the factors that may be causing image bleed-through the use of a real-time detection mechanisms while actively printing has been limited or non-existent. For example, if bleed-through was visually detected for a given media type, the print process critical parameters would be manually adjusted prior to printing the customer job and would not be adjusted during the printing of the job. Depending on the familiarity of the printer operator with the print process, the adjustment may or may not ultimately alleviate the bleed-through condition.